Abstract wrote:We perform a dynamical analysis of available observations gathered during imaging campaigns by Marois et al. (2008). The preliminary orbital solutions of this three-planet system involving massive objects of 10 Jupiter-masses each lead to strongly unstable configurations. Although the semimajor axes are large (about of 24, 36 and 68 AU, respectively), the planets are heavily interacting and their orbits are located in a zone spanned by numerous low-order mean motion resonances. Constraining the initial conditions by the available imaging observations and the requirement of dynamical stability (the Copernican Principle), we search for the long-term stable configurations, living at least for a time comparable with the parent star life-time (100 Myr). We also test a hypothesis of yet undetected fourth planet that could be hidden in the coronagraph images and that could stabilize the whole system.

Abstract wrote:Three planets have been directly imaged around the young star HR 8799. The planets are 5-13 Mjup and orbit the star at projected separations of 24-68 AU. While the initial detection occurred in 2007, two of the planets were recovered in a re-analysis of data obtained in 2004. Here we present a detection of the furthest planet of that system, HR 8799 b, in archival HST/NICMOS data from 1998. The detection was made using the locally-optimized combination of images algorithm to construct, from a large set of HST/NICMOS images of different stars taken from the archive, an optimized reference point-spread function image used to subtract the light of the primary star from the images of HR 8799. This new approach improves the sensitivity to planets at small separations by a factor of ~10 compared to traditional roll deconvolution. The new detection provides an astrometry point 10 years before the most recent observations, and is consistent with a Keplerian circular orbit with a~70 AU and low orbital inclination. The new photometry point, in the F160W filter, is in good agreement with an atmosphere model with intermediate clouds and vertical stratification, and thus suggests the presence of significant water absorption in the planet's atmosphere. The success of the new approach used here highlights a path for the search and characterization of exoplanets with future space telescopes, such as the James Webb Space Telescope or a Terrestrial Planet Finder.

Abstract wrote:The discovery of three planetary companions around HR 8799 (Marois et al. 2008) marked a significant epoch in direct imaging of extrasolar planets. Given the importance of this system, we re-analyzed H band images of HR 8799 obtained with the Subaru 36-elements adaptive optics (AO) in July 2002. The low-order AO imaging combined with the classical PSF-subtraction methods even revealed the extrasolar planet, HR 8799b. Our observations in 2002 confirmed that it has been orbiting HR 8799 in a counter-clockwise direction. The flux of HR 8799b was consistent with those in the later epochs within the uncertainty of 0.25 mag, further supporting the planetary mass estimate by Marois et al. (2008).

Abstract wrote:The recent discovery of a three-planet extrasolar system of HR 8799 by Marois et al. is a breakthrough in the field of the direct imaging. This great achievement raises questions on the formation and dynamical stability of the HR 8799 system, because Keplerian fits to astrometric data are strongly unstable during ~0.2Myr. We search for stable, self-consistent N-body orbits with the so called GAMP method that incorporates stability constraints into the optimization algorithm. Our searches reveal only small regions of stable motions in the phase space of three-planet, coplanar configurations. Most likely, if the planetary masses are in 10-Jupiter-mass range, they may be stable only if the planets are involved in two- or three-body mean motion resonances (MMRs). We found that 80% systems found by GAMP that survived 30Myr backwards integrations, eventually become unstable after 100Myr. It could mean that the HR 8799 system undergo a phase of planet-planet scattering. We test a hypothesis that the less certain detection of the innermost object is due to a blending effect. In such a case, two-planet best-fit systems are mostly stable, on quasi-circular orbits and close to the 5:2 MMR, resembling the Jupiter-Saturn pair.

This paper investigates the possibility of more companions to HR 8799.

Abstract wrote:The extrasolar planetary system around HR 8799 is the first multiplanet system ever imaged. It is also, by a wide margin, the highest mass system with >27 Jupiters of planetary mass past 25 AU. This is a remarkable system with no analogue with any other known planetary system. In the first part of this paper we investigate the nature of two faint objects imaged near the system. These objects are considerably fainter (H=20.4, and 21.6 mag) and more distant (projected separations of 612, and 534 AU) than the three known planetary companions b, c, and d (68-24 AU). It is possible that these two objects could be lower mass planets (of mass ~5 and ~3 Jupiters) that have been scattered to wider orbits. We make the first direct comparison of newly reduced archival Gemini adaptive optics images to archival HST/NICMOS images. With nearly a decade between these epochs we can accurately assess the proper motion nature of each candidate companion. We find that both objects are unbound to HR 8799 and are background. We estimate that HR 8799 has no companions of H<22 from ~5-15 arcsec. Any scattered giant planets in the HR 8799 system are >600 AU or less than 3 Jupiters in mass. In the second part of this paper we carry out a search for wider common proper motion objects. While we identify no bound companions to HR 8799, our search yields 16 objects within 1 degree in the NOMAD catalog and POSS DSS images with similar (+/-20 mas/yr) proper motions to HR 8799, three of which warrant follow-up observations.

More on the HR 8799 debris disc. Three components: an inner warm belt at 6-15 AU, an outer disc at 90-300 AU, and a halo at 300-1000 AU. This halo suggests the outer parts of the disc are dynamically active. A similar phenomenon is observed at Vega.

Abstract wrote:We present a pre-discovery H-band image of the HR 8799 planetary system that reveals all three planets in August 2007. The data were obtained with the Keck adaptive optics system, using angular differential imaging and a coronagraph. We confirm the physical association of all three planets, including HR 8799d, which had only been detected in 2008 images taken two months apart, and whose association with HR 8799 was least secure until now. We confirm that the planets are 2-3 mag fainter than field brown dwarfs of comparable near-infrared colors. We note that similar under-luminosity is characteristic of young substellar objects at the L/T spectral type transition, and is likely due to enhanced dust content and non-equilibrium CO/CH_4 chemistry in their atmospheres. Finally, we place an upper limit of 18 mag per square arc second on the >120 AU H-band dust-scattered light from the HR 8799 debris disk. The upper limit on the integrated scattered light flux is 1e-4 times the photospheric level, 24 times fainter than the debris ring around HR 4796A.

That's going to have implications for the stability calculations: one of the suggestions for ensuring the system is stable over the age of the star is that HR 8799d is not associated with the system/spurious detection. That doesn't work any more...

Well it depends on the mass of the brown dwarf: at the low end of the mass range they cool quite quickly (especially since such objects do not fuse lithium). According to this comparison, various models allow for quite high masses.

On the other hand the configuration of the system (a non-hierarchical system with low-eccentricity orbits, with inner and outer debris belts) argues against the brown dwarf classification, even if some of these objects are in the deuterium-fusion regime. There are several discoveries which suggest the high mass tail of the planetary distribution includes objects that fuse deuterium at some point in their life cycle, while the low-mass tail of the stellar (brown dwarf) distribution includes objects which do not undergo internal fusion at all.

Probably on a much smaller and denser scale. I'm not entirely sure about this, so correct me if I'm wrong. The Oort cloud is expected to extend out several tens of thousands of AU. While HR 8799 may have such an Oort cloud, it's probably far too faint to detect, and what we are seeing is the protoplanetary disk that has been scattered outward. Perhaps given sufficient time, the planets will scatter much of what we see further out, adding to HR 8799's Oort cloud and subtracting from the visible disk.

Sirius_Alpha wrote:Probably on a much smaller and denser scale. I'm not entirely sure about this, so correct me if I'm wrong. The Oort cloud is expected to extend out several tens of thousands of AU. While HR 8799 may have such an Oort cloud, it's probably far too faint to detect, and what we are seeing is the protoplanetary disk that has been scattered outward. Perhaps given sufficient time, the planets will scatter much of what we see further out, adding to HR 8799's Oort cloud and subtracting from the visible disk.

this looks like therefore rather the scattered disc (Éris and co.) and in the cloud of Hills (Sedna...)